Realisation of ultra-low loss photonic crystal slab waveguide devices

In this paper we demonstrate low loss transmission both above and below the primary band-gap for a photonic crystal (PC) super-prism device consisting of 600 lattice periods. By modifying the refractive index of the holes, we reduce overall insertion loss to just 4.5 dB across the entire visible spectrum. We show that the remaining loss is predominantly due to impedance mismatch at the boundaries between patterned and unpatterned slab waveguide regions. Experimental loss measurements compare well with finite difference time domain simulations

Spectral evolution of femtosecond pulses in nonlinear waveguides: measuring continuum generation and group velocity with NSOM

We describe spectral measurements using NSOM of the nonlinear propagation of femtosecond pulses in waveguides. Nonlinearity produces self phase modulation, which broadens the spectrum of the propagating light as it travels along the guide. This process is the basis for supercontinuum sources that span octaves in frequency, which are important for metrology[1] and carrier-envelope phase measurement[2]. Our experiments uniquely measure the spectral evolution of this continuum as the light propagates along the waveguide by sampling the evanescent field with an uncoated NSOM tip, allowing measurement of the spectrum as it develops over long (mm) length scales, and also allowing the study of the submicron evolution of the spectra both along and across the guide. The evolution in one example is shown in figure 1(a). We will compare the measured continuum generation with our nonlinear propagation modeling, and discuss how the observed spectra differ from predictions of simple models

Ultrabroadband transmission measurements on waveguides of silicon-rich silicon dioxide

We report ultrabroadband measurements on waveguides of photoluminescent silicon-rich silicon dioxide produced by plasma enhanced chemical vapor deposition. Material absorption below 700 nm and waveguide loss above 1300 nm leave a broad spectral region of good transmission properties, which overlaps with the photoluminescence spectrum of the core material. Proposed mechanisms for the material absorption and photoluminescence are discussed based on our findings

Visible wavelength super-refraction in Photonic Crystal Superprisms

We demonstrate the fabrication of superprism devices in photonic crystal waveguides with excellent transmission through 600 rows of 160nm diameter holes. Broadband spectral and angular measurements allow mapping of the chromatic refractivity. This shows the ability of such devices to super-refract by more than 1°/nm close to the principal band gaps,10× more than equivalent gratings, and 100× more than equivalent prisms. Simple theories based on plane-wave models give excellent agreement with these results

Visible photonic bandgap engineering in silicon nitride waveguides

We demonstrate experimentally the tuning of complete photonic band gaps in patterned silicon nitride waveguides. Transmission measurements were performed using an ultrabroadband high-brightness white light laser continuum, extracting extinction ratios as low as 10–4 in the gap regions. Angle-resolved measurements show the perfect alignment of the gap around the Γ-J direction

Experimental investigation of photonic crystal waveguide devices and line-defect waveguide bends

Photonic crystal waveguide devices incorporating line-defect waveguide bends have been fabricated. In this paper we present preliminary experimental analysis of these structures. Although evidence of photonic band-gap effects are observed in the spectra, transmission efficiency was found to be extremely low due to significant up-scattering losses from the holes. In order to quantify this loss mechanism, a detailed experimental and theoretical analysis of scattering effects in regular photonic crystal waveguide devices with band gaps at visible wavelength is presented. Field profiles in line defect structures are analysed using a FDTD (finite difference time domain) method

Tuning localized plasmon cavities for optimized surface-enhanced Raman scattering

Mesostructured metallic substrates composed of square pyramidal pits are shown to confine localized plasmons. Plasmon frequency tuning is demonstrated using white light reflection spectroscopy with a wide range of structure dimensions from 400 to 3000 nm. Using a simple plasmon cavity model, we demonstrate how the individual pit morphology and not their periodicity controls the resonance frequencies. By measuring the surface-enhanced Raman scattering (SERS) signals from monolayers of benzenethiol on the same range of mesostructures, we extract a quantitative connection between absorption, field enhancement, and SERS signals. The match between theory and experiment enables effective design of plasmon devices tailored for particular applications, such as optimizing SERS substrates. © 2007 The American Physical Society.This work was supported by EPSRC NanoPhotonics Portfolio EP/C511786/1.Peer Reviewe

Complete and absolute photonic bandgaps in highly symmetric photonic quasicrystals embedded in low refractive index materials

It is firmly established that periodic lattice structures can support photonic bandgaps (PBG). However, complete and absolute photonic bandgaps (CAPBG) have only been achieved in high dielectric constant mediums such as GaAs (ε=13.6). An artificial quasiperiodic photonic crystal based on the random square-triangle tiling system was designed and fabricated. The photonic quasicrystal possesses 12-fold symmetry and was analysed using a finite difference time domain (FDTD) approach. High orders of symmetry in photonic quasicrystals have been shown to provide isotropic bandgaps across all the directions of propagation of light. As an outcome of these properties, this new class of photonic quasicrystal has been shown, for the first time, to possess a secondary non-directional CAPBG for a relatively low index material, silicon nitride (ε=4.08). These materials are compatible with integrated optical technologies. This allows the fabrication of efficient integrated optical PBG devices such as WDM filters and multiplexers to become a real possibility

Self-phase modulation induced spectral broadening of ultrashort laser pulses in tantalum pentoxide (Ta₂O₅) rib waveguide

Self-phase modulation induced spectral broadening has been observed for ultrashort pulses propagating through Ta2O5 rib waveguide. The associated nonlinear refractive index was estimated to be 7.23 x10-19 m2/W, which is higher by one order of magnitude than silica glass